Method for controlling the formation of a fiber web of a fiber or paper producing process

ABSTRACT

A method for controlling the formation of a fiber web of a fiber or paper producing process includes a plurality of successive individual method steps in which controllable chemical and/or physical sequences or process steps are carried out in dependence on measured values. The materials required for the formation of the fiber web are treated, combined, and/or dewatered in the sequences or process steps. At least some of the measured values are detected inline and directly or indirectly used in order to control the formation. At least the manipulated variables, which influence the formation in a relevant manner, of the individual sequences or processes are formed dependent on definable secondary conditions during the entire process.

The invention relates to a method for controlling the formation of afiber web of a fiber or paper-producing process.

Fiber webs inter alia may also be a tissue or a cardboard web.

The process of fiber or paper production substantially is composed of aplurality of successive individual method steps in which controllablechemical and/or physical sequences or process steps, respectively, takeplace depending on measured values. In this fashion, the materialsrequired for generating the fiber web are treated, combined, and/ordewatered in the individual process steps. The measured values may bedetected inline and directly or indirectly used for controlling theformation. Besides, the measured values may however also be determinedoffline in a laboratory.

A particularly important measured variable in judging the quality of thefiber web is the formation, that is to say the distribution andcomposition of the fibers in the web. By employing powerfulquality-measuring technology it is possible to obtain an exact onlineevaluation of the structure and the uniformity of the internal fiberdistribution (paper formation) in the paper. With this, further qualityparameters, such as printability, surface finish, strength, rigidity,optical quality, etc. may be improved.

The formation is influenced by various modifiable essential variables ormanipulated variables, respectively, such as vacuum, wire tension, etc..However, each re-adjustment of a manipulated variable not onlyinfluences the process said manipulated variable is intended toinfluence, but also downstream processes. Re-adjustments of a processthus cause consequential effects, such as, for example, increased wirewear, higher usage of chemicals, etc.. These effects thus always alsoaffect the total costs.

A number of publications pertaining to controlling the formation areknown from the prior art. In this fashion, a method for optimizing theformation by modifying the headbox feed consistency by way of the lipopening is known, for example. Further controls are disclosed in EP 1454 012 A1 or WO 00/34575.

It is one of the objects of the invention to propose a formation controlwhich improves the formation in the fiber web.

It is a further object of the invention to provide a method forcontrolling the formation, which enables the operation of the papermachine to be stabilized.

According to the invention a method for controlling the formation of afiber web of the type mentioned at the outset is proposed, in which atleast the manipulated variables of the individual sequences orprocesses, respectively, which influence the formation in a relevantmanner in the entire process are generated depending on definablesecondary conditions.

The formation is influenced by a plurality of successive individualmethod steps in which controllable chemical and/or physical sequences orprocess steps, respectively, take place depending on measured values,wherein at least some of the measured values are detected inline anddirectly or indirectly used for controlling the formation.

The materials required for generating the fiber web are treated,combined, and/or dewatered in successive individual method steps. Inorder for the entire process to be controlled, all measured values areprocessed in a data processing system, and manipulated values accordingto defined specifications are generated therefrom.

In turn, consequential effects which are undesirable arise when thecontrol values are re-adjusted. In this fashion the formation may indeedbe improved by re-adjusting a control value, but increased wire wear mayarise as a consequential effect on account thereof, for example when thevacuum is set to be too high.

If, as proposed, the manipulated values of the individual sequences orprocesses, respectively, in the entire process are generated dependingon definable secondary conditions, negative effects of this type may beprevented.

Secondary conditions in the sense of the invention are thus understoodto be conditions which defines value ranges which must not be departedfrom or which permit a re-adjustment of the control values only within adefined range, respectively, such that the measured values at themeasuring points which are assigned to the process do not overshootand/or undershoot certain limits.

Furthermore, the manipulated variables may be generated depending ondefinable costing functions. One consequential effect may be the costs.In this fashion the formation or the entire process, respectively, maybe additionally optimized with a view toward reducing costs, wherein atall times also the other secondary conditions and ultimately also theformation has to be within certain limits.

In this fashion, the costs of expenditure which arise as a result of themodification of the manipulated variables may also be evaluated by meansof the costing function. Furthermore, the costing function however mayalso evaluate the consequential costs.

The secondary condition may be included in generating the limit valuesas an equality condition, for example formation=constant, or else as aninequality, for example increasing vacua in the wire station along thedewatering section, such as p1>p2>p3.

Furthermore, the secondary conditions may be generated depending on theinitial materials or raw materials, respectively, and/or on thechemicals, auxiliaries, and energy supplied in the successive methodsteps, as well as on the materials and emissions to be disposed of.

In order to minimize the costs, an optimizing algorithm by means ofwhich the costing functions may be optimized while adhering to thesecondary conditions may be employed, in that, while considering thesecondary conditions and the consequential effects, all decisivemanipulated variables are only re-adjusted to the extent that theformation achieves a target value or a formation value, respectively.

In this fashion the optimizing algorithm, in order to adhere to thesecondary formation conditions, may influence the manipulated variablesinfluencing the individual sequences or processes, respectively, in theentire process such that the value of a potential deviation of theformation from the nominal value is minimized.

On the other hand, the optimizing algorithm may also be assigned astored model which either by way of a-priori knowledge or by way ofinterpretation of the effects of previous re-adjustments reproduces theinfluence of the manipulated variables on the formation in a qualitativemanner, on account of which the down times of the processes areadvantageously conjointly considered.

However, both procedures may also be combined with each other, such thatfurther optimization takes place.

The individual methods in the sense of the invention substantially takeplace in the stock preparation, the headbox feed, and the wet end of afiber-web production machine; that is to say in those regions of aproduction machine for fiber webs where modifying or influencing theformation may take place.

In this fashion, at least one of the following manipulated variables maybe used for controlling the formation in the stock preparation:

-   -   type of retention agent    -   dosing point of retention agent    -   amount of retention agent    -   grinding performance    -   dispersion performance    -   material composition    -   amount of fixing agent.

In this fashion, at least one of the following manipulated variables maybe used for controlling the formation in the headbox feed:

-   -   suspension jet geometry    -   lip opening    -   aperture position    -   lamella position    -   inserts position    -   speed differential between jet and wire.

Furthermore, at least one of the following manipulated variables may beused for controlling the formation in the wet end:

-   -   dewatering strip geometry    -   dewatering strip pressures    -   vacuum    -   wire tension.

However, the wire characteristics as well as the wire running time, inparticular the change in the CFM value, have an influence on the stablerunning of the machines, as well as on the costs, and may be included inthe secondary conditions as a function, for example.

In this fashion it is, however, also possible for the formation to beoptimized in that the machine speed is modified, on account of which therisk of web rupturing is reduced, in particular in the case of rawmaterials which are difficult to process or else in the case of variableclimatic conditions. Web rupturing has a great influence on the totalcosts.

It is one of the particular advantages of the invention that operationalstability and the formation can be stabilized in such a manner that thecosts of the entire process can be reduced to an optimal minimum.

Further features of the method according to the invention and furtheradvantages of the invention are derived from the following descriptionwith reference to the drawing.

The invention will be explained in more detail in the following by meansof diagrams, in which:

FIG. 1 shows a block diagram for illustrating the formation control,

FIG. 2 shows a line chart for illustrating the correlations betweenmanipulated variables, secondary conditions, and costs relative to aconstant formation.

FIG. 1 shows a block diagram for illustrating the formation control withthe aid of which functioning of the system or of the control,respectively, of the formation may be described.

The system or the control 1, respectively, of the formation of a fiberweb of a fiber or paper producing process depends on a multiplicity ofsuccessive individual method steps. In this fashion, variouscontrollable chemical and/or physical sequences or process steps,respectively, take place in the individual methods steps, depending onmeasured values, in order to treat, combine, and/or to dewater thematerial required for the formation of the fiber web.

The individual method steps which are responsible for generating theformation have been compiled in FIG. 1 in block 3. The methods orprocesses may take place in the stock preparation, the wet end process,the headbox feed, and the former, wherein each process is capable ofbeing influenced by at least one manipulated variable 2. Referring tothe possible manipulated variables a1, a2, . . . , reference is made tothose already mentioned, this not being a complete enumeration.

Besides the manipulated variables, the secondary conditions have aninfluence on the formation in that individual relevant manipulatedvariables are generated depending on definable secondary conditions.

The secondary conditions are defined in such a manner that aparticularly stable operation of the paper machine is ensured. Certainmeasured values thus must not exceed certain limits which must bemandatorily adhered to in order for the formation to be optimized.

The control strategy may be implemented with the aid of an optimizingalgorithm which minimizes the costing function and thereby adheres tothe secondary conditions. These secondary conditions may be present asan equality condition (for example, formation=constant), as limit values(control limits, for example, 0.9<jet-wire ratio<1.1) or as aninequality (increasing vacua along dewatering, for example, p1>p2>p3).

The consequential effects 5 are derived from the individualre-adjustments of the manipulated variables of the processes. Theconsequential effects may be measured online or in the laboratory, andare directly or indirectly included in the secondary conditions. Inother words, the limits of the secondary conditions are influenced bythe consequential conditions. Consequential effects may include wear,energy consumption, consumption of chemicals, etc..

A line diagram for illustrating the correlations between manipulatedvariables, secondary conditions, and costs relative to a constantformation is illustrated in FIG. 2.

The essential variables (expenditure) which are re-adjustable by thecontrol are evaluated as expenditure costs by way of a correspondingpricing function. The consequential costs which arise on account of there-adjustable essential variables are likewise determined. The costingfunction thus calculates from the total cost from the expenditure andconsequential costs of the setting of the manipulated variables whichare relevant to the formation. This costing function is minimized by wayof an optimizing algorithm, while adhering to the secondary conditionsdefined above, such that an (iterative) step-by-step modification of thesetting takes place up to a cost-optimized operating point, wherein theformation remains as a variable within a permissible tolerance range.

In order to adhere to the secondary conditions of the formation, theoptimizing algorithm does/can design re-adjusting such that the value ofa potential deviation of the formation from the nominal value (range) isminimized (in an ideal case to 0). This may take place by way of a modelwhich either by way of a-priori knowledge or by way of interpretation ofthe effects of the previous re-adjustments reproduces the influence ofthe manipulated variables on the formation in a quantitative manner.

LIST OF REFERENCE SIGNS

-   1 Block diagram-   2 Manipulated variables-   3 Method steps-   4 Secondary conditions-   4 a Limit values of secondary conditions for manipulated variable Y-   4 b Limit values of secondary conditions for manipulated variable X-   5 Consequential effects-   6 Target value of formation-   8 Range of validity-   9 Cost expenditure

1-15. (canceled)
 16. A method for controlling the formation of a fiberweb in a production process (fiber or paper-production), the methodcomprising: performing a plurality of successive individual method stepsand thereby acquiring measured values; carrying out controllablechemical and/or physical sequences or process steps, respectively, inwhich materials required for generating the fiber web are treated,combined, and/or dewatered in dependence on the measured values, andthereby detecting at least some of the measured values inline anddirectly or indirectly using the measured values for controlling theformation; and generating at least manipulated variables of theindividual sequences or processes, respectively, which influence theformation in a relevant manner in the entire process in dependence ondefinable secondary conditions.
 17. The method according to claim 16,which comprises generating the manipulated variables depending on adefinable costing function.
 18. The method according to claim 17,wherein the costing function evaluates a cost of expenditures arising asa result of a modification of the manipulated variables.
 19. The methodaccording to claim 17, wherein the costing function evaluatesconsequential costs arising as a result of a modification of themanipulated variables.
 20. The method according to claim 16, wherein oneof the secondary conditions is an equality condition.
 21. The methodaccording to claim 16, wherein one of the secondary conditions is aninequality.
 22. The method according to claim 16, which comprisesgenerating the secondary conditions depending on the initial materialsor raw materials, respectively, and/or on the chemicals, auxiliaries,and energy supplied in the successive method steps, as well as on thematerials and emissions to be disposed of.
 23. The method according toclaim 17, which comprises minimizing the costs with the costing functionby way of an optimizing algorithm while adhering to the secondaryconditions.
 24. The method according to claim 23, wherein the optimizingalgorithm, in order to adhere to the secondary formation conditions,influences the manipulated variables influencing the individualsequences or processes, respectively, in the entire process so as tominimize a value of a potential deviation of the formation from anominal value.
 25. The method according to claim 23, which comprisesassigning the optimizing algorithm a stored model which, either by wayof a-priori knowledge or by way of interpretation of effects of previousre-adjustments, reproduces an influence of the manipulated variables onthe formation in a qualitative manner.
 26. The method according to claim16, which comprises implementing the individual methods substantially ina stock preparation, a headbox feed, and at a wet end of a fiber-webproduction machine.
 27. The method according to claim 26, whichcomprises using at least one of the following manipulated variables inthe stock preparation for controlling the formation: a type of retentionagent; a dosing point of retention agent; an amount of retention agent;a grinding performance; a dispersion performance; a materialcomposition; and an amount of fixing agent.
 28. The method according toclaim 26, which comprises using at least one of the followingmanipulated variables in the headbox feed for controlling the formation:a suspension jet geometry; a lip opening; an aperture position; alamella position; inserts position; and a speed differential between jetand wire.
 29. The method according to claim 26, which comprises using atleast one of the following manipulated variables at the wet end forcontrolling the formation: a dewatering strip geometry; dewatering strippressures; a vacuum; and a wire tension.
 30. The method according toclaim 16, wherein one of the manipulated variables is a machine speed.